Considering that you are diagnosed with OCD, your psychiatrist has prescribed you with SSRI initially. While you need to ask this particular question to your doctor, the possible reasons for shifting you to a new medication would be any adverse effect on the initial medication, new information in your history that shows you might show better response to the new medication or when it is seen that you are not having any clear response to the first medication.
However, you should go ahead and discuss these issues with your doctor and also get a second opinion through an individual consultation if you still have any queries.
Next Steps
Discuss with your doctor